1. Field of the Invention
The present invention relates to position encoders, and more particularly, to a multi-track encoder and method for measuring absolute position for position sensing applications.
2. Description of Related Art
Position sensors and precise measurement devices generally require a detector that converts relative motion or position of two elements into an electrical signal. The electrical signal is then processed to determine the position and/or displacement.
Existing diffractive optical encoders for position sensing use the interference pattern from a diffractive grating to produce sinusoidal signals on a detector inserted in the fringe pattern. The sinusoidal signals are then electronically interpolated by an interpolator to detect the position and/or displacement.
The present invention is a multi-track absolute encoder, hereinafter referred to as a xe2x80x9cMulti-Dimensional Encoderxe2x80x9d or MDE, and a method for measuring absolute position, using two or more periodic tracks (e.g., diffractive gratings) of slightly differing periods to generate fringe patterns on an appropriately designed multi-track sensor.
The present invention is a position sensing device having a plurality of signal generators that provide signals having different periods in response to movement. A first processing circuitry extracts a position phase from each periodic signal and a second processing circuitry combines the position phases to estimate absolute position.
According to one embodiment of the present invention, the diffractive grating tracks are positioned to be illuminated by a light source. Two or more detectors are positioned to detect the interference fringes from the corresponding diffractive grating tracks. A first processing circuitry, such as an interpolator, is coupled to the detectors for obtaining phase signals from the detectors. The interpolator is coupled to a second processing circuitry, such as a track combiner processor, which is responsive to a combination of the phase signals and identifies the fringe count of one or more of the grating tracks from which the combination of phase signals is estimated to have originated.
The number of tracks and their periods are selected so that the two or more measured phases form a unique vector (in the mathematical sense) for each position in the measurement range. That is, the periods of the grating tracks are selected to be different such that the combination of phase signals from the interference fringes as the grating tracks are moved relative to the detectors defines unique combinations of phases over a range of movement. By mapping this unique vector back into fringe count, the grating displacement can be estimated to within one fringe. Since the output of any one of the tracks is suitable for fine position sensing within each fringe, the combination of the fringe count and the fine phase gives an absolute position reading without any memory of prior position.
The absolute encoder of the present invention is similar to current encoders manufactured by MicroE of Natick, Massachusetts. However, the present invention incorporates additional diffractive grating tracks, a multi-channel detector, which is a detector with parallel linear arrays which act like MicroE""s standard xe2x80x9cphased arrayxe2x80x9d detector, and a special processing algorithm to convert the multiple measured phases into a single absolute position estimate.